High carbon hot rolled steel sheets using high carbon steel have been used in various applications, e.g., machine parts, tools, automobile parts, and the like. Such steel sheets, suitable for the above-described applications, are manufactured by forming hot rolled steel sheets having corresponding target thicknesses, performing blanking, bending and press-forming on the hot rolled steel sheets to obtain desired shapes, and finally performing a heat treatment process on the hot rolled steel sheets to impart high hardness to the hot rolled steel sheets.
High carbon hot rolled steel sheets may require excellent material uniformity because high material deviations in the high carbon hot rolled steel sheets not only worsen dimensional precision in a forming process and cause defects during processing, but also lead to non-uniform structure distribution even in a final heat treatment process.
Although various inventions have been suggested to improve the formability of high carbon hot rolled steel sheets, most inventions have only focused on controlling the sizes and distribution of carbides in microstructures after a cold rolling process and an annealing process, no invention regarding the formability and heat treatment uniformity of hot rolled steel sheets has been proposed.
More specifically, patent document 1, related to the formability of a high carbon annealed steel sheet obtained after performing cold rolling and annealing discloses that the formability of the steel sheet is improved if a carbide distribution, in which an average carbide particle diameter is 1 μm or less and a fraction of carbides having a particle diameter of 0.3 μm or less is 20% or less, is obtained by controlling annealing conditions. However, there is no mention of the formability of a hot rolled steel sheet. Moreover, carbides do not necessarily have to be formed to have a particle diameter of 1 μm or less after annealing a hot rolled steel sheet having excellent formability.
Further, even in patent document 2 in which a ferrite particle diameter of 5 μm or more and a carbide particle diameter standard deviation of 0.5 or less are prescribed by properly controlling annealing conditions, there is no mention of hot rolled structure, and a hot rolled steel sheet having excellent formability does not necessarily have to maintain the same carbide distribution as in the above-mentioned invention after being treated under ordinary annealing conditions.
Patent document 3 discloses that fine blanking workability increases when ferrite grain sizes satisfy a range of 10 μm to 20 μm while maintaining fractions of pearlite and cementite to levels of 10% or less. Although the disclosed invention specifies the controlling of the microstructure of an annealed steel sheet, the formability of the disclosed invention is far from that of a hot rolled structure. On the contrary, as a method of improving the formability of a hot rolled structure, if the formation of ferrite is suppressed and a uniform phase distribution is obtained, material deviations may be minimized.
Patent document 4 suggests a hot rolled structure-prescribing method of obtaining a ferrite fraction of about 10% or less by adjusting a ferrite particle diameter to be 6 μm or less after annealing and a carbide particle diameter to be within the range of 0.1 μm to 1.2 μm after annealing, and cooling a hot rolled steel sheet at a rate of 120° C. per second or higher. However, the disclosed invention is for improving stretch-flangeability of an annealed steel sheet, and a fast cooling rate of 120° C./sec is not always required to form a hot rolled steel sheet having a ferrite fraction of about 10% or less.
Patent document 5 suggests a method of improving the formability of an annealed steel sheet by adjusting fractions of pro-eutectoid ferrite and pearlite to be 5% or less respectively, forming a high carbon bainite structure having a bainite fraction of 90% or more, and forming a structure in which fine cementite is distributed after annealing. However, the disclosed invention is only for improving the formability of an annealed steel sheet by finely adjusting an average carbide size to be 1 μm or less and a grain size to be 5 μm or less, but is not related to the formability of a hot rolled steel sheet.
(Patent document 1) Japanese Patent Application Laid-open Publication No. 2005-344194
(Patent document 2) Japanese Patent Application Laid-open Publication No. 2005-344196
(Patent document 3) Japanese Patent Application Laid-open Publication No. 2001-140037
(Patent document 4) Japanese Patent Application Laid-open Publication No. 2006-063394
(Patent document 5) Korean Patent Application Laid-open Publication No. 2007-0068289